Perhaloalkanoic acids having the formula EQU A--(CFX--CFY).sub.n --CFX--COOH
in which A is a perfluoromethyl, perfluoroethyl, perfluoropropyl or perfluoroisopropyl radical, X is a fluoro or a trifluoromethyl radical, Y is a fluoro or chloro radical, and n is an integer from 1 to 4, and mixtures of such perhaloalkanoic acids, are valuable intermediates for the production of compounds that are surfactants.
Starting materials for the production of these perhaloalkanoic acids or mixtures thereof are perhaloalkanes having the formula EQU A--(CFX--CFY).sub.n --CFX--CFYZ
in which A, X, Y and n have the same significance as in the foregoing formula and Z is an iodo, chloro or bromo radical which perhaloalkanes can be produced, for example, by telomerization of polyhaloolefins such as tetrafluoroethylene with a taxogen having the formula EQU CF.sub.3 --CF.sub.2 I
if the perhaloalkans have the formula EQU CF.sub.3 --CF.sub.2 --(CF.sub.2 --CF.sub.2).sub.1-4 --CF.sub.2 --CF.sub.2 I
By reaction with oxidizing agents these telomeric perhaloalkanes are converted to perhaloalkanoic acids such as perfluoroalkanoic acids or perfluoroalkanoic acids in which two or more of the fluoro substituents have been replaced by chloro substituents. This oxidation can be effected in an acidic medium, for example, in oleum in the presence of a catalyst, as well as in an alkaline medium with oxygen or an alkali-metal permanganate. In either case, mixtures containing perhaloalkanoic acids and unconverted perhaloalkanes are obtained.
The separation of such reaction mixtures into their components is usually beset with considerable difficulties, especially in the case of mixtures that were prepared by oxidation of mixtures containing perhaloalkanes of various chain lengths. The resulting mixtures contain not only the various perhaloalkanoic acids but also unconverted perhaloalkanes of various chain lengths. Because of the overlapping of their boiling points and vapor pressures, a satisfactory separation and purification of the perhaloalkanoic acids and perhaloalkanes from each other cannot be obtained by fractional distillation of such mixtures. Furthermore, vacuum distillation of the perhaloalkanoic acid mixture is also not a suitable procedure for this purpose since, dependent upon the method of preparation of the mixture, water and alkali-metal salts of the perfluoro-alkanoic acids which can produce vigorous foaming during the distillation may be present therein as impurities. Since the alkali-metal salts produce decomposition of perhaloalkanoic acids at elevated temperatures into olefins, carbon dioxide and alkali-metal halides, the presence of alkali-metal salts in such mixtures during their distillation may decrease the yield of the desired perhaloalkanoic acids.
Separation of such mixtures of perhaloalkanoic acids and perhaloalkanes from each other and purification of such mixtures of perhaloalkanoic acids by extraction with solvents is also difficult because of the notorious tendency of the perhaloalkanoic acids as well as their salts to form emulsions, especially when extracted with water, since the acids as well as their alkali-metal salts form stable emulsions with water that are broken only with difficulty. Such emulsions also produce considerable difficulty when the perhaloalkanoic acid is extracted from an aqueous solution thereof with an organic solvent.
Perhaloalkanoic acids as such are very readily soluble in many organic solvents as are, to some extent, salts of such acids with metals and non-metals. Furthermore, perhaloalkanoic acids are themselves excellent solvents for the unconverted perhaloalkane telomers. Because of this, it is not possible to attain by extraction with organic solvents that are normally used for extractions a complete separation of the perhaloalkanoic acids from the perhaloalkanes from which they were produced.
In order to obviate these difficulties it was proposed in U.S. Pat. No. 2,904,566 granted to Edgar Fischer and German Pat. No. 1,047,764 to distill the crude acid with superheated steam while introducing gaseous hydrogen chloride or hydrogen bromide together with the steam into the distilling apparatus to prevent decomposition of the perhaloalkanoic acid. The cost of the apparatus that is required for such treatment may be high since, if metal apparatus is used, its corrosion by the hydrogen halide must be taken into consideration. Since perhaloalkanoic acids are to some extent good solvents for water, additional operating costs are required in this process to obtain anhydrous perhaloalkanoic acids.
A process is described in German Pat. No. 1,058,492 for the separation of perhaloalkanoic acids from water-insoluble components thereof, particularly long-chain perhaloalkanes, by use of formic acid containing up to 90% and preferably between 0 and 50% by weight of water. In order to prevent boiling, the extraction is performed at superatmospheric pressure and temperatures under 100.degree. C. A disadvantage of this process is that costly pressure apparatus is required.